There has been significant progress in understanding how various physiologic parameters, such as coronary flow, perfusion pressure, diastolic pressure and influent oxygen tension, affect local myocardial pO2 as measured by EPR oximetry with lithium phthalocyanine. This in turn has allowed measurements of local oxygen tension, oxygen consumption and left ventricular work to be used for the determination and study of additional physiologic parameters, such as capillary density, and to test various models of coronary flow. It has also led to better control of experimental factors. Additional experiments have been performed to study the stability of oxygen tension in both the constant flow and constant pressure variants of the isolated perfused heart. Effects on myocardial pO2 of pharmacological interventions, such as nitric oxide which is known to play an important role in endothelial physiology, is providing new and important data. Studies using no flow ischemia show a significant increase in myocardial pO2 and coronary perfusion pressure, without significant change in oxygen consumption,left ventricular work or efficiency after brief repetitive ischemic episodes. Presumably, increased myocardial pO2 after repetitive ischemia represents increased oxygen delivery via the microcirculation due to redistribution of microvascular resistance. EPR oximetry continues to advance our understanding of myocardial oxygen levels and cardiac physiology.